Punch and dies have been used for decades by metal fabricators as a common process for creating holes in metal plate. The die usually has a surface and above defining a cutting edge upon which the metal plate is positioned. The male punch element which, moves generally perpendicular relative to the female die and metal plate thereon, is concentrically aligned with the die bore. The punch is pressed through the steel plate and into the bore, creating a hole in the plate and cutting a slug from the metal plate material. The slug should either be frictionally retained inside the die or the slug should drop off the punch face so the punch can recycle to produce another hole and slug in a subsequent cycle of the punch press.
Slug retention or slug pulling as it is commonly known is a significant problem with such tool and die apparatus. Slug pulling will cause machine down time as well as material, tool and machine damage. Slug pulling occurs when the slug does not separate from the punch face, but actually gets pulled fully or partially up by the punch and out of the die as the punch cycles on an up-stroke. A number of factors can cause slug pulling. A lubricant is usually used to reduce wear and keep the punch and die sets in good condition as well as to reduce the tonnage required to punch a hole. These lubricants can create a vacuum effect between the flat face of the punch and the top of the slug. Lighter oils i.e. generally oils of lower viscosity, may reduce the vacuum effect to some extent, but slug pulling still occurs. Furthermore, lighter oils vaporize and are messy. Also, as the punch begins to wear, a raised butt is created on top of the slug that can “hug” or wrap itself around the punch adhering to the punch to cause slug pulling. It is also possible for the punch to become magnetized, thus causing an undesired adherence of the slug to the slug face.
Tool and die manufacturer's solutions generally use the concept of trying to retain the slug in the die by use of friction. By way of example, if the slug is squeezed in the bore hole of the die tight enough, the slug friction will be greater than the vacuum between the slug and the punch face on the up stroke of the punch cycle. Known friction die add internal vertical ridges or slightly off vertical ridges on the walls of the die bore, i.e., ridges or ribs which run substantially parallel with the longitudinal axis of the bore hole. Other known devices utilize protrusions in the bore, for example a tapered pressure point or points. None of these solutions have been shown to be particularly effective as slug pulling can still occur, and as the internal ridges or pressure points wear, the slug pulling problem gets worse.